Method of fabricating silicon-on-insulator substrate

ABSTRACT

A method of fabricating a SOI substrate is disclosed, which includes the steps of: forming trenches in the Si substrate; forming an oxidation preventing film over the Si substrate and at the side-walls of the trenches; forming grooves at the bottoms of trenches by etching the Si substrate using the oxidation preventing film as a mask; carrying out an oxidation using to form an oxide film and a Si device layer isolated by the oxide film; removing the oxidation preventing film; and carrying out a planarization to form the silicon-on-insulator substrate having a planar surface.

BACKGROUND OF THE INVENTION

The present invention relates to a silicon-on-insulator ("SOI")substrate and method of fabricating the same, and more particularly to aSOI substrate and method of fabricating the same including a Si devicelayer having a constant thickness.

In general, in a fabrication process of a complementary metal oxidesemiconductor ("CMOS") transistor, an isolation region of a large areashould is needed in order to isolate devices and prevent latch-up of aCMOS transistor. There are, however, problems in that an isolationregion of a large area results in a reduced chip dimension and a reducedintegration of devices.

A SOI technique had been proposed in order to above problems. Withcomplete isolation between devices, a SOI substrate, having a buriedoxide film sandwiched between a Si handling substrate and a Si devicesubstrate, prevents latch-up of a CMOS transistor and allows for highoperational speed of devices.

One fabrication method of the SOI substrate is a separation by implantedoxygen ("SIMOX") method. In a conventional SIMOX technique, referring toFIG. 1A, there is a Si substrate 1 doped with impurity ions. Oxygen ionsare implanted into the substrate 1. Referring to FIG. 1B, an annealingis carried out to form a buried oxide film 2 and a Si device layer 1A inthe Si substrate 1. Referring to FIG. 1C, a field oxide 3 is formed inthe Si device layer 1A by a LOCOS, thereby forming a SOI substrate 100.

However, the conventional SIMOX technique has a disadvantage in thatdislocation in a surface of a Si layer easily occurs when implantingoxygen ions, thereby generating a great quantity of leakage current.Therefore, it is difficult to control a thickness of the Si layer wherea device is to be formed.

Another method of fabrication is a bonding technique. In conventionalbonding technique, referring to FIG.2A, there are provided a Si devicesubstrate 10 and a handling substrate 11. A buried oxide film 12 isformed on the handling substrate 11 by a thermal oxidation. Referring toFIG. 2B, the handling substrate 11 and the device substrate 10 arebonded, with the buried oxide film 12 existing between substrates 10 and11. Afterwards, most of the device substrate 10 is etched by grindingand lapping and then the ground and lapped device substrate 10 ischemical and mechanical polished to high degree of precision, therebyforming a Si device layer 10A. Referring to FIG. 2C, an isolation film13 is formed in the Si device layer 10A to define an active region,thereby forming a SOI substrate 200 having a Si device layer 10A.

According to the conventional bonding technique, it is difficult toprecisely control a polishing stop point in chemical and mechanicalpolishing to form the Si device layer 10A. Since, the thickness of theSi device layer 10A is not constant, yield of semi-conductor devices isreduced. In addition, the conventional bonding technique hasdisadvantages of a complicated fabrication process and high fabricationcost. Furthermore, the conventional techniques have a shortcoming inthat a separate additional process, which forms a field oxide in a Sidevice layer, is required in order to define an active region.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of fabricatinga SOI substrate having a Si device layer where the surface is notdamaged.

Another object of the present invention is to provide a method offabricating a SOI substrate with a simplified process.

Another object of the present invention is to provide a method, offabricating a SOI substrate, which simultaneously forms an isolationfilm and a buried oxide film.

In accordance with one embodiment, there is provided a method offabricating a SOI substrate, comprising the steps of: forming trenchesin the Si substrate; forming an oxidation preventing film over the Sisubstrate and at the side-walls of the trenches; forming grooves at thebottoms of trenches by etching the Si substrate using the oxidationpreventing film as a mask; carrying out an oxidation to form an oxidefilm and a Si device layer isolated by the oxide film; removing theoxidation preventing film; and carrying out a planarization to form thesilicon-on-insulator having a planar surface.

In addition, there is also provided a SOI substrate including a Sisubstrate; a Si device layer formed over the Si substrate to isolate Sidevice layer from the substrate, the insulating layer being coplanarwith the device layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and feature of the invention may be better understood withreference to the following detailed description, appended claims, andattached drawings wherein:

FIG. 1A through FIG. 1C are sectional views illustrating a process offabricating a SOI substrate according to a conventional SIMOX technique;

FIG. 2A through FIG. 2C are sectional views illustrating a process offabricating a SOI substrate according to a conventional boningtechnique;

FIG. 3A through FIG. 3F are sectional views illustrating a process offabricating a SOI substrate in accordance with one embodiment of thepresent invention;

FIG. 4 is a sectional view illustrating a process of fabricating a SOIsubstrate in accordance with another embodiment of the presentinvention; and

FIG. 5 is a sectional view illustrating a process of fabricating a SOIsubstrate in accordance with further another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 3A, in accordance with one embodiment of the presentinvention, trenches T are formed by an anisotropic etching method at theportion of the Si substrate 20 where an isolation film is to be formed,the Si substrate being doped with impurity ions.

Referring to FIG. 3B, silicon nitride layer 21, which serves as a layerfor preventing oxidation, is deposited to a predetermined thickness overthe Si substrate 20 including trenches T. A photoresist pattern 22 isformed by depositing a photoresist layer and then exposing portions ofthe silicon nitride layer 21 existing over the bottoms of the trenches Tby a conventional photolithography method. Herein, a pad oxide (notshown) may be formed between the Si substrate 20 and the silicon nitridelayer 21 in order to reduce stress due to a difference in coefficientsof thermal expansion between the Si substrate 20 and the silicon nitridelayer 21.

Referring to FIG. 3C, the resultant structure is then anisotropicetched, removing the photoresist pattern 22 and the exposed portions ofthe silicon nitride layer 21, thereby exposing the Si substrate 20 atthe bottom of the trenches.

Referring to FIG. 3D, the exposed Si substrate 20 is isotropic etched byusing the silicon nitride layer 21 as a mask to form grooves H at abottom of trenches T. Furthermore, the exposed Si substrate 20 at thebottom of the trenches T is side-etched from the isotropic etchingmethod, etching an bottom area wider than that of the original trenchesto thereby narrow the distance between adjacent trenches T.

Referring to FIG. 3E, the Si substrate 20 is thermal-oxidized to form anoxide film 23 and a Si device layer 20A which is isolated by the oxidefilm 23.

Referring to FIG. 3F, the silicon nitride layer 21 and the Si devicelayer 20A are etched away until the surface of the oxide film 23 isexposed, thus removing the silicon nitride layer 21. The etching is doneby chemical and mechanical polishing or etching back to form a SOIsubstrate 300 having an planar surface. Herein, the portion 23A of theoxide film 23 existing between adjacent Si device layer 20A serves as anfield oxide for an isolation of a SOI substrate. In addition, theportion 23B of the oxide film 23 existing in the Si substrate 20underlying the Si device layer 20A, serves as a buried oxide of the SOIsubstrate. According to the present invention, the buried oxide 23B anda field oxide 23A for an isolation are simultaneously formed.

Referring to FIG. 4, in accordance with another embodiment of thepresent invention, in order to hasten a thermal oxidation for formingthe oxide 23, a polysilicon layer 30 is formed at the grooves H in thebottom of trenches T, before thermal oxidation of FIG. 3D. If thermaloxidation is then carried out, with using the polysilicon layer 30 assilicon providing layer, the oxide film 23 is formed at a thicknesssufficient to approach at the surface of the Si substrate 20 as shown inFIG. 3E.

Referring to FIG. 5, in accordance with further another embodiment ofthe present invention, before thermal oxidation of FIG. 3D, oxygen ionsare implanted into the Si device substrate at the grooves H to hasten athermal oxidation.

According to the present invention, a field oxide for an isolation and aburied oxide are simultaneously formed by a thermal oxidation usingtrenches and then the surface of a Si substrate is etched back orchemical and mechanical polished, thereby forming a SOI substrate.Accordingly, the SOI substrate is not damaged with exclusion of an ionimplantation for forming a buried oxide in SIMOX as well as afabrication process is simplified.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications of the illustrative embodiments,as well as other embodiments of the invention, will be apparent topersons skilled in the art upon reference to this description. It istherefore contemplated that the appended claims will cover any suchmodifications or embodiments as fall within the true scope of theinvention.

What is claimed is:
 1. A method of fabricating a silicon-on-insulatorsubstrate, comprising the steps of:forming trenches in a Si substrate,said trenches each having a side-wall and a bottom; forming an oxidationpreventing film over said Si substrate and the side-walls of saidtrenches; forming grooves at the bottoms of said trenches by etchingsaid Si substrate using said oxidation preventing film as a mask;oxidizing said grooves to form an oxide film and a Si device layercompletely dielectrically isolated by said oxide film; and planarizingby etchback, thereby removing the oxidation preventing film and exposingthe oxide film.
 2. The method as claimed in claim 1, wherein saidoxidation preventing film is silicon nitride.
 3. The method as claimedin claim 1, wherein said step for forming said oxidation preventing filmincludes the step for:depositing a silicon nitride over said Sisubstrate including said trenches; forming a photoresist pattern toexpose said silicon nitride over the bottom of said trenches; patterningsaid silicon nitride using said photoresist pattern to form saidoxidation preventing film; and removing said photoresist pattern.
 4. Themethod as claimed in claim 3, wherein said step for forming saidoxidation preventing film further includes a step for forming a padoxide over said Si substrate including said trenches, before the stepfor depositing said silicon nitride.
 5. The method as claimed in claim1, further comprising a step for forming a polysilicon layer at saidgrooves between said groove formation step and said oxidation step. 6.The method as claimed in claim 1, further comprising a step forimplanting oxygen ions into the Si substrate underlying grooves betweensaid groove formation step and said oxidation step.
 7. The method asclaimed in claim 1, wherein said planarization step is carried out bychemical and mechanical polishing.
 8. The method as claimed in claim 1,wherein the step of forming grooves is carried out by isotropic etching.